Conventional flexible materials which have been used for medical purposes include materials mainly comprising polyvinyl chloride having blended therein a plasticizing agent selected from phthalic acid-based compounds such as dioctyl phthalate and 2-ethylhexyl phthalate in an amount of from 10 to 100% by weight per 100% by weight of the polyvinyl chloride; materials mainly comprising an elastomer resin such as a styrene-butadiene-styrene-based resin, for example an ABA-type block copolymer, an ethylene-propylene copolymer, a polyester elastomer or a polyurethane elastomer; and flexible resins such as an ethylene-vinyl acetate copolymer and an ethylene-ethylacrylate copolymer.
Most of the flexible medical members fabricated from such materials, for example, blood bags, tubes, catheters and the like, are disposable products, and after their use, they are abandoned as waste materials. These materials, however, do not undergo environmental degradation and retain their original shape for a prolonged period of time. It is well known that such waste materials have induced various social problems including the pollution.
In order to solve such problems, various investigations have been recently carried forward to develop biodegradable materials, namely, high molecular weight materials capable of being decomposed in ecosystem when placed or abandoned in the environment, and these materials have attracted a considerable public attention.
Of the conventionally known high molecular weight biodegradable materials, those comprising polypropylene, polyethylene or the like having blended therein corn starch for the purpose of their morphological collapse can not be deemed essentially biodegradable, since these materials only experience morphological change with the lapse of time, and the high molecular weight backbone of the polypropylene or polyethylene do not undergo any degradation.
Another group of biodegradable materials known in the art are poly(3-hydroxybutyrate) and copolymers mainly comprising the poly(3-hydroxybutyrate). Poly(3-hydroxybutyrate) is a material which has been confirmed to undergo a considerable environmental degradation, and to have an excellent biocompatibility. Therefore, this material was highly expected to have various applications in medical and other fields.
Contrary to such expectations, the poly(3-hydroxybutyrate) failed to find a large number of applications due to insufficiency in its impact resistance and other physical properties because of its hardness and brittleness. The poly(3-hydroxybutyrate) is also poor in its workability since it undergoes decomposition in the vicinity of its melting point in spite of its useful thermoplasticity.
In view of such conditions, various attempts have been made to modify the physical properties of the poly(3-hydroxybutyrate). Japanese Patent Application Kokai No. 63(1988)-269989 discloses a copolymer comprising recurring structural units of D-(-)3-hydroxybutyrate and D-(-)3-hydroxyvalerate. This material has attained considerable improvements in reducing melting point and in increasing flexibility. Synthesis of this material, however, could be effected only at a low productivity, and also, required a special substrate for the fermentation. As a consequence, this copolymeric material was rather expensive to detract from its availability as a general-purpose material.
Other attempts of altering the physical properties of the poly(3-hydroxybutyrate) include modification of the poly(3-hydroxybutyrate) by mixing with such resin materials as polyethylene oxide, ethylene propylene rubber, and polyvinyl acetate. None of the attempts, however, have fully succeeded in providing the stability, cost performance, workability, and the like with the resulting resin compositions. Use of such resin compositions for medical applications would be even more difficult since such applications would further require high safety and hygienic properties.